Pumps of the reciprocating type have the inherent advantage of pumping the same known quantity of fluid during each operating cycle, this quantity being simply determined by the pumping chamber volume displaced by a piston. The pumping rate of such pumps can be controlled with considerable accuracy by controlling the frequency with which the pumping cycle is repeated. Accordingly, electrically operated reciprocating fluid pumps are well suited for use as rate pumps, i.e., pumps that are employed to pump fluid at a precisely controlled quantity per unit of time.
Although electrically driven reciprocating rate pumps are generally satisfactory, there are situations in which they are not suitable for use. For example, some rate pumps are operated in highly explosive environments where the presence of electrical equipment is undesirable, perhaps even totally prohibited. In some situations, pneumatic pumps are driven by compressed natural gas instead of air, making the presence of electrical equipment a potential hazard for that reason. It may then become necessary to rely on pneumatically operated pumps that do not so readily facilitate accurate control of the pumping rate.
Some hydraulic pumps, such as that described in U.S. Pat. No. 3,179,409 to Hill, have been converted to a hybrid type of apparatus in which the driving force is provided pneumatically but the pump is timed and controlled by an electrical circuit that causes a pneumatic control signal to be applied to a valve that in turn causes the pump to begin the next cycle of its operation. Purely pneumatic rate pumps have been built in which the air that provides the driving force is interrupted periodically, but the size of such pumps is severely limited because of the inability of the pneumatic controls to handle large volumes of air.
An objective of the present invention is to provide a pneumatically driven pump that is rate controlled by an entirely pneumatic mechanism.